In many diagnostic and therapeutic applications, there is great need to objectively quantify the optical density, shape and size of various ocular tissue, such as the crystalline lens and cornea. Regarding the crystalline lens, it is well known that the presence of a cataract at particular locations on the crystalline lens can effect the visual acuity and function of the eye. It is also known that the optical density of the cataract is related to the amount of light diffusion caused by increased size and coagulation of protein molecules in the crystalline lens, and that the more posterior the cataract the greater its affect on vision.
For years, prior art descriptions of cataracts have generally consisted of a morphologic statement, such as, nuclear, cortical, or posterior subcapsular. Such morphologic descriptions have been based primarily on the patient's potential visual acuity estimated using an acuity scope.
Prior art techniques in practice today are discussed in the paper "The Objective Assessment of Cataract," by Nicholas A. P. Brown. Among such prior art techniques is the use of a slit-lamp microscope for subjectively observing the visible features of the crystalline lens, which are recorded using the Oxford Clinical Cataract Classification and Grading System.
Recently, several different methods of photodocumentation and grading of lens opacities have been described for clinical and epidemiological use. For example, in the paper entitled "The Multi-Purpose Camera: A New Anterior Eye Segment Analysis System" published in Vol. 22 of Ophthalmic Research (1990), Saski et al. disclose a system for forming Scheimpflug slit images and retroillumination images of the lens using a Xenon flash lamp, a slit projection system and a CCD camera. In the paper entitled "Evaluation of Photographic Methods for Documentation of Lens Opacities," published in Investigative Ophthalmology and Visual Science, Vol. 31, No. 6, Jun. 1990, Lee et al. disclose a method of forming photographic slit and retroillumination images of the lens using the Oxford Cataract Camera from Halofax, of Hereford, England, and the Neitz Cataract Camera from Kowa Optical, of Torrance, Calif.
In general, while these prior art methods and systems can produce images of crystalline lens, both techniques and resulting images suffer from significant shortcomings and drawbacks.
In particular, with such prior art slit-lamp photo-imaging techniques, the depth of field and focus of these systems are severely limited and, therefore, permit focused illumination of only on the center of a lens nucleus while all other surrounding structure is illuminated with diverging unfocused light. Also, with prior art slit-lamp photo-imaging systems, neither the slit width or the luminance of the light beam can be maintained uniformly constant from image to image, or photo-examination session to photo-examination session. Consequently, comparative results cannot be made from one image to another to determine if the optical density of the lens is worsening in response to drug exposure, or disease process.
Furthermore, prior art slit-lamp photo-imaging techniques are incapable of providing accurate cross-sectional images of the eye which have correct spatial relationships between ocular structures. Also, such images are characterized by reflections occurring at optical interfaces within the eye which often distort important structural features.
Thus, there is great need for a method and apparatus that is capable of producing in vivo images of ocular tissue in a way which is free from the shortcomings and drawbacks accompanying the prior art.
Accordingly, it is primary object of the present invention to provide a method and apparatus for in vivo imaging and analysis of ocular tissue in an objective, quantitative manner.
It is a further object of the present invention to provide such a method and apparatus, from which cross-sectional images of ocular tissue can be formed over a high depth of field extending far beyond the thickness of the crystalline lens.
A further object of the present invention is to provide such a method and apparatus, from which accurate cross-sectional images of ocular tissue can be formed, with correct spatial relationships between ocular structures.
A further object of the present invention is to provide a method and apparatus for precisely measuring the physical dimensions of ocular structures and their correct spatial relationships within the eye.
An even further object of the present invention is to provide a method and apparatus for forming cross-sectional images of ocular tissue which enable precise localization of zones of increased optical density, such as cataract formation, in various locations of the crystalline lens.
Yet a further object of the present invention is to provide a laser-based ocular tissue analysis system in which cross-sectional digital images of the crystalline lens and surrounding ocular structures can be formed and from which the precise degree and location of optical density of the crystalline lens can be objectively determined using digital image analysis.
An even further object of the present invention is to provide such an ocular tissue analysis system in which the luminance and cross-sectional dimension of the laser illumination used to visualize the lens and form cross-sectional ocular images, can be maintained essentially uniformly constant from image to image, and photo-examination session to photo-examination session.
An even further object of the present invention is to provide such an ocular tissue analyzing system which includes a microscope and an image detector that uses laser illumination for visualizing and forming perfectly focused cross-sectional images entirely through the ocular tissue comprising the cornea and crystalline lens.
These and other objects of invention will become apparent hereinafter and in the claims.